Current work is focused on purification and regulation of cGMP phosphodiesterase in bovine brain. The cGMP phosphodiesterase activity being studied is activated in the presence of calcium and calmodulin and may be modulated by other factors as well. Recent advances in the production of monoclonal antibodies, using specialized cell fusion techniques, permit the isolation and characterization of extremely specific antibodies to complex antigens. Such antibodies made in response to highly purified phosphodiesterase preparations are useful for pursuing various investigations including enzyme localization and regulation on the cellular level.